Air Motion Transformer
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The Air Motion Transformer (AMT) operates on a different armature principle than both dynamic and electrostatic drivers. Known also the AVT or Air Velocity Transformer, the AMT moves air in an augmented, semi-perpendicular motion using a folded Mylar sheet, structured around a series of aluminium struts positioned in a high intensity magnetic field. Although it is possible to operate the diaphragm without the magnetic field using piezoelectric technology, this method is rarely employed.
The diaphragm pushes back and forward from itself in a similar physical motion pattern to what is observed when an accordion is squeezed in and out to pump air though the reed chambers, albeit over an exceedingly smaller motion range.
To illustrate as best as can in text, the driver moves thusly:
1. /\/\/\/\/\/\/\/ 2. ||||||||||||| 3. \/\/\/\/\/\/\/\ 4. ||||||||||||| 5. /\/\/\/\/\/\/\/ 6. etc
The result is a dipole driver with an extraordinarily rapid response rate, caused both by the extremely low mass of the Mylar driver, and by the far smaller motion range it undergoes on each “swing” compared to an dynamic driver. In this technical respect, it shares characteristics with the electrostatic driver.
The discernible motion of each of these diaphragm flexes is very small, but because of the folded structure more air is moved than would be by a conventional cone or electrostatic driver of the same plotted surface area. As a matter of surface comparison, a standard one-inch-wide AMT strip has a functional driver area which is comparable to a circle-shaped dynamic cone with a diameter of eight inches. The folded driver design, combined with the small motive range, means that the AMT acts like a point source version of a larger driver, inherently resulting in lower distortion in sound reproduction. As a result of its motion pattern, the AMT "spits" the air out in a way which is compatible with what happens when, although you apply only a small amount of pressure to a slice of fruit, the pip inside shoots out at high speed. The motion of the air as it leaves the diaphragm is in fact around five times faster than the motion of the actual driver structure. Hence the name Air Motion Transformer.
The most common use for the AMT driver in consumer electronics today is as a midrange-tweeter or tweeter in high-end multi-driver speakers, sometimes paired with horns, or in the case of Precide's speaker products, with an upward-firing woofer driver. Precide's own loudspeaker models are very faithfully based upon the original designs of Dr. Oskar Heil. In the tweeter market the AMT must compete against electrostatic, ribbon, and dynamic tweeters. While apparently very good as a mid-tweeter or full tweeter, the AMT is not commonly employed as much as one may expect lower down in the speaker market. This is mainly as a corollary of their dipole sound radiation, which makes enclosure in traditional speaker cabinets difficult without sacrificing sound quality or employing sound reflex baffles. The AMT can also reproduce sound right down into the low midrange fully competently, making design decisions regarding crossover points difficult.